Although this invention relates generally to the art of mixing liquids and is applicable to areas outside the particular art of steel manufacture, it has a special application in the metallurgical techniques, and for this reason the present section deals with the background relating to steel metallurgy.
In the art of steel metallurgy, a great deal of attention is paid to the desirability and the degree of reduction of the content of two particular elements in the steel, namely sulphur and oxygen. These two elements have a tendency to combine with iron or alloying elements present in the steel to form nonmetallic inclusions.
Inclusions have significant influences on various properties of the finished product. Although almost all product properties are influenced in some way by these nonmetallic inclusions, the most harmful effects are found in relation to steel impact resistance and shelf energy. It has been proven that the reduction of sulphur to a low content (0.003 to 0.010 percent) in combination with a low oxygen content (0.002 to 0.005 percent) will produce a steel with a limited number of nonmetallic inclusions. The remaining inclusions, especially if their chemistry is controlled by the addition of certain elements such as rare earth elements, will have harmless effects on steel properties.
In the steelmaking plant the refining of steel is performed within steelmaking furnaces (open hearth, basic oxygen, electric arc, or induction furnaces). The time required for refining restricts the furnace productivity, since the furnace cannot be used for melting and other necessary operations during the refining time. This invention, by using the ladle as a primary vessel for steel refining, is directed to increasing the productivity and decreasing the operating costs of a steelmaking shop. A steelmaking ladle is generally idle during normal shop operation, but is a much cheaper vessel than the furnace.
This invention could also prove advantageous in a steelmaking shop which uses high sulphur pig iron. The steel tapped from the steelmaking furnace could have a high sulphur content which can subsequently be reduced considerably by the process of this invention as it applies to steelmaking. High sulphur pig iron can be produced in the blast furnace more efficiently than low sulphur pig iron, since the reduction of stone addition will result in an increase in blast furnace productivity, a decrease in specific energy input per ton of metal and a lower slag volume. These benefits are expected to produce savings far greater than the cost of operating the refining installation in the steelmaking shop.
This invention also is directed to increasing the yield of alloying elements, particularly those with high affinity to oxygen. Additives such as rare earth silicides or other alloys are added to a highly deoxidized metal and slag system in accordance with this invention, and loss of the additives to oxygen is significantly reduced. A reduction of the amount of alloying elements is thus permitted, which brings about material savings which can be much higher than the cost of operating the process herein disclosed.
The process of this invention, in its particular application to steel matallurgy, is based on the fact that the composition of the slag exerts a significant and important influence on the metallurgical reactions between the steel and the slag. The extent of such reactions is a function of the physico-chemical laws and the mathematical laws of the kinematics of reaction and is accordingly limited. Nevertheless, the deoxidation and desulphurization of steel by slag which is treated according to a preferred form of the process of this invention has been found to be very efficient. In this preferred form, the process of this invention includes the controlling of the slag composition, so as to control in turn the interaction between the slag and the steel.
The process of this invention, in accordance with its preferred form particularly applicable to steel metallurgy, involves influencing the chemical reactions in the molten steel by bringing about an increase of the interface area between the steel and slag on the one hand, and by influencing the physical interactions between the slag and the already formed nonmetallic inclusions on the other hand. In addition to the foregoing, the properties of the slag may be controlled by additives placed on the slag surface, either continuously during the process itself, or prior to the beginning of the process.
It has been known for some time that the desulphurization capability of steelmaking slags is related to the slag oxidizing potential (FeO, MnO, Cr.sub.2 O.sub.3, P.sub.2 O.sub.5 content) and to its basicity (CaO, MgO, SiO.sub.2, Al.sub.2 O.sub.3 content). The slag oxidizing potential can be reduced by the addition of additives with a high affinity for oxygen. For example, silicon will react with FeO and MnO forming SiO.sub.2, which is very stable and readily soluble in the slag. Metallic silicon is very expensive, but its alloys are highly suitable and economical. Ferro-silicon and calcium-silicon are two of the many such alloys. Various other materials such as lime, silica, magnesia, alumina, spar, and others can be mixed in various ratios to create a beneficial slag treatment mixture. Any slag treatment mixture should be based on some knowledge of the slag chemistry before slag treatment, and on a good estimation of the optimum slag composition after the treatment.
The oxygen level of the metallic bath is a function of the slag oxidizing potential. A low oxygen content in the bath can be obtained by bringing about a low oxygen content in the slag, and also a sufficiently high interface area between the slag and the steel. The process of this invention in its preferred form particularly suited to steel metallurgy contemplates bringing about both of these conditions.
Recent developments in physical metallurgy, together with certain economic aspects of modern designing techniques, have resulted in tighter specifications for steel quality. One of the more important factors in steel quality is identified as steel cleanliness. Various processes for obtaining improved steel cleanliness have been patented, and some of them are used on a production scale. These processes may be divided generally into four main groups:
1. Steel treatment by an artificial molten or unmolten slag placed on the ladle bottom prior to tapping of the furnace. PA1 2. Vacuum degassing and alloying under vacuum. PA1 3. Injection of desulphurizing additives or mixtures of artificial slag components with desulphurizing additivies under the steel surface. PA1 4. Increase of the steel slag interface using PA1 1. High capital investment and/or high operating costs. PA1 2. Air pollution problems. PA1 3. Small effect on steel cleanliness.
a. Induction mixing PA2 b. Mechanical stirring.
Although some of the processes belonging to these groups have proven themselves in production conditions, they are attended by certain disadvantages, among which are the following: